The present invention relates to nut and panel assemblies, particularly nut and panel assemblies used in mass production applications, such as automotive applications wherein the nut is permanently attached to the panel and the assembly is later painted or coated. In such applications, the screw hole is masked to prevent entry of the coating material. The present application discloses a unique masking plug which prevents entry of the coating material and serves as a seal if the nut is not filled with a bolt during later assembly.
In order to mount various accessories and auxiliary equipment in automotive applications, various types of nuts are attached to vehicle parts made of sheet metal, such as the side sil, bumper reinforcement, dashboard, trunk lid, hood, pillar, door, cross member, floor panel and the body components of the vehicle. Typically, weld nuts, clinch nuts or pierce nuts are used. As is generally known, weld nuts are attached to the vehicle part by resistance welding or projection welding and pierce nuts are attached to the vehicle part by an installation head and die button which punches the pilot portion of the pierce nut through the metal panel and then deforms the panel metal and sometimes the nut to mechanically interlock the nut and the panel. Clinch nuts are similarly applied, except the nut is attached in a preformed panel opening.
Vehicle parts with nuts attached are fed to the coating process independently or coated after attachment to the vehicle part. Automobile manufactures generally require a high quality film coating by the time coat-three bake method, including undercoating (primer), middle coating and top coating, which are all applied to the automobile part with the nut attached. Normally, the undercoating is applied by the electrodeposition coating method, preferably by the cationic electrodeposition coating method and electrodeposition of the coating material is completed only after the part with the nut attached has been degreased. Then, the coating is baked and hardened at temperatures of 180.degree. to 210.degree. C. after washing and dewatering to form a uniform film coating having superior rust prevention properties. The middle coating is normally applied by the spray coating or powder coating method and the coating is baked at temperatures of 160.degree. to 170.degree. C. The top coating may then be applied by the spray coating method and baked at temperatures of 160.degree. to 170.degree. C.
A first problem experienced with preattached nuts coated as described above is that it may be difficult to insert the bolt into the screw hole of the nut, when mounting various accessories, because the coating material has infiltrated into the screw hole of the nut and affixed to the threads in each of the coating steps described. The film coating in the screw hole of the pierce, clinch or weld nut makes the operation of screwing in the outer threads of the bolt into the inner threads of the screw hole difficult, increasing the torque required to tighten the bolt.
In particular, the electrodeposition coating methods described above have great throwing (transliteration) power so that the coating material adheres even in the corners of the threads of the pierce, clinch and weld nuts during the electrodeposition coating and the film coating is hardened by baking, forming an obstacle to tightening of the bolt.
Therefore, conventionally, the bore or screw hole of the nut is masked with a masking rubber or masking tape at the coating plant. Normally, masking is accomplished by manually blocking each screw hole of each nut with masking tape or filling the screw hole with a masking rubber in a manual operation. After coating, the masking tape or masking rubber is removed similarly by manual operation. Although welding of the weld nuts and mounting of pierce and clinch nuts have become automated and the electrodeposition coating process has been automated, resulting in substantial savings and labor costs, the masking operation is still dependent upon manual labor. This is due in part to the fact that the nut is mounted to the sheet metal in differing locations for vehicles to be coated and the fact that it is impossible or difficult to automate the masking operation (including removal of masking tape or masking rubber) at the coating plant. Therefore, a very substantial number of man days are necessary for masking by manual labor and the masking cost accounts for a considerable part of the total cost of coating. Further, the undercoating, middle coating and top coating may be applied at different plants and thus the masking and removal process may be required after each coating application at each plant, such that the total masking cost substantially increases the cost of the parts.
Various solutions have been suggested for masking automobile parts with nuts during the coating operation. For example, a lid formed of a thin plate material, such as metal plate, vinyl sheet or paper, has been attached at one end of the screw hole. However, such masking means is effective with respect to spray coating only. A major portion of the screw hole is left exposed and thus this method cannot be used in the case of electrodeposition coating because the parts to be coated are immersed in the immersion coating type of the electrodeposition bath and the coating particles are electrically absorbed on the parts to be coated. The throwing power of an electrodeposition bath is great and the coating material is absorbed evenly on the exposed surfaces of the screw hole in the electrodeposition bath.
The use of a foam plug has also been suggested, wherein a foamable material is inserted into the screw hole of a weld nut and the screw hole is then filled by foaming the material with the heat of welding. In this process, the heat is indispensable for foaming the foam material and therefore this process would not be suitable for pierce and clinch nuts which are attached without generating heat, as with weld nuts. The prior art also suggests filling the threaded bore of a weld nut with a sublimation solid, such as naphthalene. A sublimation solid is not sublimated during the electrodeposition coating, but is sublimated in the heat of the drying/baking step. According to this method, it is necessary to fill the screw hole with the sublimation solid only after welding to prevent the solid from being sublimated by the heat generated in the weld attachment of the weld nut to the metal panel. Consequently, this method is not suitable for mass production. Also, sublimation of naphthalene can be completed only once, such that the nut must be filled each time a coating is applied and baked, which would require three applications of the sublimation solid where a three coat-three bake coating method is used.
Electrodeposition coating of vehicle parts has yet to solve the above-described masking problem, although electrodeposition coatings of automobiles has been used for over fifteen years.
A second problem with the utilization of pierce, clinch and weld nuts is related to the production of many models of automobiles and trucks in small volumes. In recent years, there has been a trend for many models of automobiles to be manufactured in smaller volumes. Normally, weld, pierce and clinch nuts are mounted to sheet metal parts to conform to as many vehicle models, as possible. Thus, many nuts are left unused, depending upon the vehicle model. The unused nuts, which do not have a bolt inserted into the threaded bore, not only generate noise created by wind when the wind passes through the nut bore, but the nut bore also allows noise and foreign matter, such as mud, to enter into the car, such as when the unused nut is in the floor panel or dashboard.
In several automotive applications, the threads of the pierce nuts are coated with a fluorocarbon polymer, such that paint does not strongly adhere to the threads during subsequent coating applications. Thus, a bolt or screw can be more easily threaded into the nut. However, the nut is not sealed during subsequent coating applications and therefore coating of the threads does not solve the second problem described above. That is, unused nuts generate wind noise and allow passage of foreign matter, as described. Thus, the prior art has failed to solve the described problems associated with the use of weld and pierce nuts.